Diets of desert mule deer in altered habitats in the lower Sonoran Desert

1. What they did

Large parts of central Sonora, Mexico, have been overgrazed by cattle and turned into pasture of buffelgrass, a grass brought in from elsewhere. By the 1990s it covered 1.2 to 1.6 million hectares of Sonoran rangeland. Nobody knew how this affects wildlife, and the diets of desert mule deer had never been described in the Sonoran Desert of Mexico. The authors set out to describe what desert mule deer eat there, and to compare the diets of mule deer and cattle where buffelgrass pastures occur.

• Study areas. Four ranches in central and western Sonora, from the wetter Plains of Sonora (320 mm of rain a year) to the dry Central Gulf Coast (180 mm). Buffelgrass covered 27% of one area, over 50% of another, and under 2% of the driest area.
• Collecting dung. At Rancho San Luis (1991 and 1992), all old pellets were cleared from 1200 permanent circular plots along 24 transects, and every mule deer pellet group that appeared was collected each month. Cattle dung was collected from plots where cattle gathered (water, resting places, salt licks). In the other three areas (2002 to 2004), at least 18 fresh mule deer pellet groups were collected each month across the area, during a radio-tracking study of the deer.
• Reading the plant fragments. In the laboratory, samples were pooled by season and year, and plant fragments in the dung were identified under a microscope against reference slides. Plants were grouped as browse (shrubs and trees), forbs (herbaceous plants), grasses and succulents (cacti).
• Comparing diets. They measured how much the diets of mule deer and cattle overlapped in each season, and how varied (diverse) the mule deer diet was in each area and season.

2. What they found

What mule deer ate

• 96 plant species: 45 browse, 33 forbs, 12 grasses and 6 succulents. 69 of these had never been reported as food for desert mule deer.
• Browse was the main food in every area, 77% to 88% of the yearly diet. Mesquite, ironwood, jojoba, bird-of-paradise and fairy duster each made up 10% or more of the diet somewhere.
• Forbs made up 5% to 10% on average, rising during or after the summer and winter rains, when annual plants appear.
• Grasses were eaten least: under 1% of the yearly diet at La Jubaivena and El Americano, 2% at La Pintada and 9% at San Luis. At San Luis, grasses made up 45% and 19% of the summer diet in 1991 and 1992, mostly native grasses.
• Buffelgrass was not important for mule deer: at most 5% of the summer diet and under 1% of the yearly diet, even though it was plentiful.
• Cacti varied most between areas, from 2% of the diet at El Americano to 12% at La Pintada. Chainfruit cholla was the cactus eaten most, found in the diet in all areas.

What cattle ate (Rancho San Luis)

• 57 plant species. Grasses made up 57% of the diet, browse 30%, forbs 9% and cacti 4.5%.
• Buffelgrass was the cattle’s most important food, 21% of the diet, eaten most in summer and autumn.
• Ironwood and mesquite, the two most important browse plants for the deer, were also the most important browse plants for cattle. Cattle ate the most browse in spring (45%).

Overlap and diversity

• Mule deer and cattle shared 45 food plants, but their diets only overlapped in a meaningful way in spring (Morisita index 0.67). Overall overlap was 0.50, below the 0.60 the authors count as overlap.
• The variety of plants in the deer diet differed only a little. The only significant difference between areas was between the wettest (San Luis) and the driest (El Americano), with the driest area having the most varied diet.

3. What they concluded

• As elsewhere in their range, desert mule deer in Sonora live mainly on browse, and their diet changes with area, season and year, following what plants are available.
• The high use of grasses at Rancho San Luis may reflect the good cover of native grasses there, but the authors say this is not conclusive and needs more research.
• Diet diversity followed rainfall: the drier the area, the more varied the diet.
• Buffelgrass, widespread in the Plains of Sonora, is little used by desert mule deer.
• Habitat altered for cattle, with fewer shrubs and trees for cover and browse, is a challenge for managing and conserving desert mule deer in northwest Mexico.

4. How tracking was or was not used

Tracking was not a focus of the study. The whole study rests on collecting dung (scat) and attributing it to mule deer or cattle: mule deer pellet groups were gathered from cleared plots and across the study areas. The paper does not say how mule deer pellets were told apart from those of other animals, although white-tailed deer, collared peccary and feral burros were also present.

5. Tracker or observer certification

Not reported. The paper does not say who collected or identified the pellet groups in the field, or how. Field assistants are thanked by name. The laboratory technician who identified plant fragments under the microscope is described as experienced and tested for reliability, but that concerns the plant identification, not the dung identification.

6. Other technologies, skills or innovations

• Microhistological analysis: identifying plant fragments in dung under a microscope (100 times magnification) against reference slides made from local plants.
• Permanent, pre-cleared pellet plots, so that each month’s collection held only fresh dung.

7. Implications for conservation (from the paper)

• Desert mule deer rely on browse, so the loss of shrub and tree cover and browse in habitat altered for cattle is a challenge for their conservation in northwest Mexico.
• Buffelgrass pastures, which now cover large areas, add little food for desert mule deer.
• Knowing the main food plants of desert mule deer, and how they relate to each other, is essential for improving habitat.